In recent years there has been considerable interest in Europa - one of
the four Galilean moons of Jupiter. The Galileo
Mission has provided encouraging evidence that Europa might have an
ocean of liquid water under a layer of ice and this has stimulated speculation
that life might possibly exist in such an environment. A future mission to send a spacecraft into orbit around Europa is still awaiting funding. (See Europa Mission: Lost In NASA Budget.) The goal would be to determine definitively if such an ocean really exists. If the result turns out to be positive, then
a subsequent mission
will send some kind of robotic
submarine to melt through the ice and explore the sea below. On this
page I will try to trace the origin of the idea that an ocean might exist
under Europa's icy crust. Then I will describe some of the early speculations
about how life might have come to evolve in such an environment.
Many of the articles, books, and lectures that will be mentioned discuss
not just Europa, but also two other Galilean moons - Ganymede and Callisto,
which in the 1970s were also thought to potentially possess an ocean of
liquid water (still a real possibility). Just to give the flavor
of the research and speculation into this topic, I will include copious
quotes, letting the authors speak for themselves. The reader will
also find numerous links to various related topics and to some of the beautiful
photos that are available on the internet (indicated by an asterisk *).Jupiter*
has sixteen known moons. The four largest are Io*,
Europa*,
Ganymede*,
andCallisto*
which were
discovered
by Galileoin 1610. Simon Marius, who may
have discovered them at the same time as Galileo, named them after Jupiter's
illicit lovers in Greek
and Roman mythology. They are usually called
the Galilean satellites and referred to in many scientific papers as JI,
JII, JIII, and JIV in the above order (which is the order of
their distances to Jupiter). Until Pioneer
10 and 11 reached the Jupiter
system* in 1973-74, astronomers studied these moons with Earth-based
telescopes. As early as 1951, the geophysicist H. Jeffreys proposed
the possibility that Callisto might be partially or totally composed of
water in the form of ice. This was suggested by Callisto's very low density,
and also its albedo.
In a lecture given in 1957 at a meeting of the American Astronomical Society,
the astronomer G.P. Kuiper discussed his study of the spectrum of reflected
sunlight from the Galilean moons which was based on observations made at
the McDonald Observatory. After mentioning a marked difference for
Europa and Ganymede, he stated that
: "This is most readily explained
by assuming that JII and JIII are covered by H2O
snow." In the mid-1960s, additional evidence that these moons
might be covered with water in some form was found by the Soviet astronomer
V.I. Moroz who wrote that : "Europa and Ganymede could very well
be covered with ice , if not entirely, at least in large part. Europa
shows the deepest ice absorptions and the lowest temperature."
This was again based on studying light spectra
[A,
B].
Then, in the early 1970s, various astronomers were able to
provide solid confirmation of the existence of water frost or ice on these
moons by analyzing the absorption of the infrared frequencies of sunlight
reflected from their surfaces.. This became possible because, at
that time, laboratory studies of the spectrum of water ice for those
frequencies had just recently been carried out. For example, in their
paper Galilean Satellites: Identification of Water Frost (in
Science,
vol. 178, 1972 ), C.B. Pilcher, S.T. Ridgway, and T.B. McCord
report the results of their measurements of infrared reflectivity observed
from the solar telescope at Kitt Peak National Observatory. They
determined that between 50 and 100% of the surface of Europa, 20
to 65% of the surface of Ganymede, and 5 to 25% of Callisto are covered
with water frost . This is one of a number of papers from the same
period which attempted to understand details about the surfaces of the
Galilean satellites by carefully examining the properties of the light
reflected from the Sun and from other sources.
The idea that Europa and other ice-covered bodies in our solar system might
possess an ocean of liquid water under a crust of ice was first proposed
by John S. Lewis in his paper Satellites of the Outer Planets: Their
Physical and Chemical Nature (which appeared in Icarus,
vol.15, 1971). It is a theoretical paper which proposes models of
the structure of various moons of the outer planets of our Solar System
based on making some simplifying assumptions and some hypotheses about
the chemical composition of these ice-covered bodies. The available
data concerning their composition was quite limited at the time and so
Lewis writes: "We must therefore depend to a considerable
extent upon our knowledge of the composition of the Sun and the chemical
behavior of volatiles
[such as H20] at low temperatures in making
plausible conjectures regarding the bulk composition of solid material
in the outer solar system." Lewis begins the paper with a
succinct summary: "Steady-state thermal models for the icy satellites
are constructed in which the energy released by radioactive
decay [A
, B]
in the interiors of the satellites is exactly balanced by the net radiative
loss from their surfaces. It is shown that the Galilean satellites of Jupiter
and the large satellites of Saturn, Uranus, and Neptune very likely have
extensively melted interiors and most probably contain a core of hydrous silicates,
an extensive mantle of ammonia-rich liquid water, and a relatively thin
crust of ices." The heart of the paper is a mathematical analysis
of the flow of heat from the core to the surface due to a phenomenon known
as convection [A,
B,
C].
This analysis is based on estimates of the surface temperature of these
icy bodies and the amount of heat which would be produced in the core by
radioactive elements. The rate of heating in the core is assumed
to be that given by the average rate of decay of uranium, thorium, and
radioactive potassium. At the end of this paper, Lewis suggests that if
an ocean exists in these ice-covered bodies, then there might be a detectable
magnetic field to search for: "An extensive electrically conducting
mantle forced to convect by an input of heat from below may be conducive
to the production of a measurable magnetic field."
In a slightly earlier article entitled Satellites of the Outer
Planets: Thermal Models (in Science, vol. 172, 1971),
Lewis announced his theory, discussing specifically Callisto:
"Steady-state thermal models for the large satellites of the outer
planets strongly indicate that their interiors are currently maintained
at temperatures well above the ice-ammonia eutectic temperature by the
decay of long-lived radioisotopes of potassium, uranium, and thorium. The
present-day steady-state thermal
structure of a representative satellite, JIV (Callisto) is shown
to be characterized by the presence of a thin icy crust over a deep liquid
mantle, with a dense core of hydrous silicates and iron oxides." A
few years later, in 1974, Lewis gave one of nine Guggenheim Lectures in
a series called Man and Cosmos at the Smithsonian Institution.
His lecture, entitled
The Outer Planets, includes a discussion of
the large satellites in our Solar System in which he describes his theory
as follows: "A mixture of icy and rocky material, assembled
into a body the size of Mercury, heats itself up by the decay of naturally
occurring radioactive elements in its interior, causing it to melt. The
dense silicates will settle to form a core, which we might think of as
being made of mud. That will leave a very thin ice crust floating upon
a thick mantle composed of a solution of ammonia in water. This kind of
structure has not been observed by spacecraft or by direct observations,
but it is a suggestion of what we may someday observe in the course of
exploring the satellites of the outer planets."
The papers of Lewis just mentioned and several other theoretical papers
that will be discussed here are based on theories about the formation
and early history of the Solar System [A,
B,
C].
Jupiter and its system of orbiting moons may have originated in a way analogous
to the formation of our Sun and its orbiting planets. The details of this
early history would be responsible for many characteristics of these moons
as they are today - their orbits, chemical composition, densities, etc.
An article entitled Implications of Jupiter's Early Contraction History
for the Composition of the Galilean Satellites by J.B.Pollack and R.T.Reynolds
was published in 1974 (in
Icarus, vol. 21) , exploring the consequences
of theories about the formation of Jupiter and its satellites. The
authors write: "Recent calculations of the gravitational contraction
history of Jupiter indicate that Jupiter's luminosity was orders of magnitude
larger during its early lifetime than it is today. As a result one might
speculate that the condensation of icy volatiles to form satellites would
be inhibited at close distances to Jupiter and compositional differences
among Jupiter's satellites might be generated. We propose that the observed
systematic variation of the mean density of the Galilean satellites with
distance from Jupiter is a result of the above circumstances."
Based on those recent calculations, they argue that the Galilean satellites
would have had an abundance of liquid water at least for millions of years
in their early history and that "water ice appears to be the
only ice likely to condense out in significant proportions, i.e. the Galilean
satellites are [presently] mixtures of rocky material and water
ice." Their theory also gives an explanation for the fact that the
densities
of the Galilean satellites decrease with their distance to Jupiter. That
is, Io has the highest density and Callisto the lowest. The
International Astronomical Union sponsored a conference at Cornell University
in 1974 called Planetary Satellites. A rather substantial volume
with the same title was published in 1977, edited by J.A.Burns, which contains
27 papers summarizing the state of knowledge in the mid-1970s concerning
the various moons in our Solar System. Most of these papers originated
as lectures delivered at that conference. A paper by G.J. Consolmagno
and J.S. Lewis, entitled Preliminary Thermal History Models of Icy Satellites,
outlines the general ideas and underlying assumptions behind the thermal
models which the authors were developing, based on the ideas in the
1971 papers of Lewis. Other papers in the volume deal with the rings
of Saturn, the atmosphere of Titan, the surfaces of some of the satellites,
theories about the formation of the outer planets and their satellites.
Several papers discuss the orbits of satellites, how the orbits evolve
over time, and the phenomenon of orbital resonance which turns out to be
especially interesting and important for the Galilean satellites. Another
paper from the Planetary Satellites conference, entitled Io's Surface
and the Histories of the Galilean Satellites, by F.P. Fanale, T.V.Johnson,
and D.L.Matson, discusses in considerable detail the possible nature
of Io's surface and how it may have come about. Observations from
Earth and from Pioneer 10 and 11 showed clearly that Io is quite different
from Europa, Ganymede, and Callisto. The authors argue that Io started
with much less water which then was drawn to the surface and may have evaporated
rather completely from that body into space leaving the surface covered
with the resulting salts: "After considering current data
and various compositional hypotheses, we conclude that Io's properties
can best be explained if it is postulated that the surface of Io is largely
covered by "evaporate" salts produced by defluidization of Io's interior,
migration of salt-saturated solutions to Io's surface and subsequent H2O
loss to space." "Io's surface seems to represent the end result
of a surface dehydration process. On Europa's surface, this dehydration
is not complete, and it appears that "clean" H2O
ice has been added to it lately at a faster rate than the rate of loss.
Ganymede and Callisto (especially Callisto) appear to have very thick (
>100 km) ice crusts overlying huge (>600 km) liquid H2O
mantles." The authors obtain these conclusions by developing
models for the thermal history of the Galilean satellites by an approach
which is somewhat different mathematically than that of Consolmagno
and
Lewis. Concerning Europa, their model suggests a much thinner
ice crust and only the possibility of a liquid water mantle. Pioneer
10arrived at the Jupiter system and started sending
back valuable data at the end of 1973. That encounter sparked an
increased interest in Jupiter and its moons and led to the planning of
a volume devoted to the latest research . The volume, which became
a 1200-page compendium called
JUPITER: Studies of the interior, atmosphere,
magnetosphere, and satellites (edited by T. Gehrels), appeared
in 1976. One long paper by A.G.W. Cameron and J.B.Pollack discusses
the origin of Jupiter and its satellites. Many of the other papers
discuss in detail various questions about Jupiter's atmosphere, ionosphere,
magnetic field, and radiation belts. More than 200 pages are devoted to
Jupiter's moons. There is a paper by Lewis and Consolmagno, entitled
Structural
and Thermal Models of Icy Galilean Satellites, which gives rather detailed
models for Europa, Ganymede, and Callisto. The paper makes various sets
of hypotheses about the early history of these satellites and the models
are based on computer simulations carried out earlier by Consolmagno.
In the conclusion of this paper, the authors suggest on the basis of their
models that at present Callisto might have a 1000-km deep liquid water
ocean covered by a 200-km thick crust consisting of rock and ice,
Ganymede might have a 400-800 km deep liquid water ocean and a 100-km thick
crust of ice, and Europa might have a 100-km deep ocean of liquid water
below a crust of ice 70-km in thickness. Here
are a few quotes from the Lewis-Consolmagno paper: "Thermal
history models are presented for a suite of possible initial structures.
Complete melting and differentiation of the ice component of Europa and
Ganymede due to internal heat sources are predicted." In the
section describing their models for Europa after various periods of time
(dating from the origin of the Jupiter system), they write: "After
250 million years, substantial melting has already taken place, resulting
in differentiation
of the water and silicates. Melting has proceeded almost to the surface,
and a crust of pure ice now exists." "After 4.5 billion years,
a structure similar to what we expect for the present may exist.
A thin crust of ice covers a convecting region of water, which is cooling
off the upper layers of the silicate core. Heat production in the
core has dropped as well, as the radioactive nuclides decay. However, the
center is still effectively isolated from the surface and continues to
heat up reaching temperatures of 2800oK." "Internal heat
sources seem to be sufficient for Europa and Ganymede to completely melt
at some time in their history (at least to within 30 km of the surface.)"
"A Europa with 10% water content would have a 70-km ice crust at present,
a 100-km water mantle, and a rocky core 1400 km in radius."
"Our models predict considerable thermal expansion, and this may produce
significant cracks in the crust, leading to upwelling of the less-dense
liquid material underneath and eventually to catastrophic overturn of the
crustal layers. But the thermal expansion appears to be on a slow enough
time scale that plastic flow of the ice should heal such cracks as they
develop." "Europa and Ganymede, with thin ice crusts as we
predict, would be more easily punctured by an impact: liquid water
could then flow from the mantle onto the surface forming a flat, clean
plain...."
The two Voyager
Missions arrived at the Jupiter system in 1979. During
that same year, three important papers were published which changed the
theoretical picture considerably. The first paper, entitled
On
the Internal Structure of the Major Satellites of the Outer Planets
by R.T.Reynolds and P.M.Cassen (in Geophysical Research Letters,
vol. 6), was written late in 1978. That paper threw some doubt on the theoretical
models of the interiors of the Galilean satellites which had previously
been proposed. The authors argue that a crust of ice which covers
a layer of liquid water and is at least 30-km in thickness would be unstable.
That is, even though the liquid water layer is heated from below
by radioactive decay in the core, it would nevertheless gradually freeze
because of the cold, icy crust above. They write: "Thermal
convection in this planetary ice layer is efficient and will solidify an
underlying liquid shell in a time that is short compared with the age of
the body." Their mathematical analysis is based on studying
solid-state convection in the ice crust. If the ice crust is thick
enough , then a quantity called the Rayleigh number will exceed a certain
critical value and that would imply that the ice crust is not stable.
The thermal models predicted by Consolmagno and Lewis would imply that
the ice crusts on Ganymede, Callisto, and Europa are thicker than 30 km.
The authors then write that: "The considerations of this investigation
apply most specifically to Ganymede and Callisto, with probable application
to Europa, Titan, and Triton, all of which are expected to contain large
fractions of H2O." Thus
it would seem that Ganymede, Callisto, and probably Europa should have
a frozen crust of ice covering a core heated by radioactive decay,
and without a mantle of liquid water in between .
However, as it turns out, there is another possible source of heat
-
the
immense gravitational force of Jupiter. Just a couple of months later,
Reynolds and Cassen together with S.J.Peale wrote a paper entitled
Melting
of Io by Tidal Dissipation which was published in Science, vol.
203, and appeared just a few days before the Voyager 1 flyby of Io on March
5th ,1979. According to Soderblum's survey article in the January,
1980 issue of Scientific American, this paper caused quite a bit
of excitement at NASA because of its surprising prediction
that there should be widespread volcanic
activity on the surface of Io. Within a few weeks, this prediction
was confirmed by studying the images
of Io*sent back by Voyager 1. The
idea is that the gravitational force on Io exerted by Jupiter should vary
enough as Io travels around Jupiter to create a strong tidal effect [A,
B,
C].
The Galilean satellites have orbits which are nearly circular. But
not exactly circular! These orbits are elliptical
and the discrepancy from being perfectly circular is measured by a number
called the eccentricity. The orbit of Io has the largest eccentricity.
It only takes 42.5 hours for Io to complete one revolution around Jupiter.
This means that Io reaches the point in its orbit which is closest to Jupiter
just slightly more than 21 hours after reaching the farthest point.
During that interval of time the gravitational force exerted by Jupiter
on Io varies by about 17% . The resulting push and pull on the surface
of Io creates frictional heat and substantial melting under the surface
and, consequently, volcanic activity. For
Europa, there should be a similar tidal effect. It takes slightly more
than 85 hours for Europa to complete one orbit. The eccentricity
of this orbit is smaller than that of Io, and Jupiter's gravitational force
on Europa will vary by about 4% during one revolution, again creating a
significant tidal effect over a very short time interval. Cassen,
Reynolds, and Peale pursue this idea in an article that was written about
one month before the Voyager 2 flyby of Europa on July 9th, 1979.
This article was entitled Is There Liquid Water on Europa and appeared
in Geophysical Research Letters, vol. 6 in September of that year.
The authors write: "It is possible that tidal dissipation
in an ice crust on Europa preserved a liquid water layer beneath it,
provided that the three-body orbital resonance
for Io, Europa, and Ganymede is ancient. The liquid water layer could be
a continuing source of the observed surface frost. If Europa's water mantle
were ever completely frozen, heating by tidal dissipation would not
exceed that produced by radioactive elements, and the mantle would remain
frozen." The phrase
"orbital resonance" refers to the fact that the period in which Io completes
one orbit around Jupiter is almost exactly half of the period in which
Europa completes an orbit, and that period is in turn almost exactly half
of the period for Ganymede to complete an orbit [A
, B].
It is this rhythm which is responsible for the eccentricity in the orbit
of Io and of Europa. The authors argue that if this orbital resonance was
present early enough in the history of Europa, then the tidal effect on
the ice crust might have generated enough frictional heat to prevent the
freezing of a liquid water mantle. They write: "But suppose
that Europa's H2O mantle was melted at
one time by some other process, perhaps during the satellite's formation.
Then heating by tidal dissipation in a growing ice crust might prevent
freezing of the entire mantle. The heating rate in the ice crust is greater
than in a completely solid body because the unsupported crust is subject
to greater deformation, even though the tidal forces are the same. With
the current eccentricity, the maximum amplitude of the variable tide on
Europa would approach 50 meters for a thin ice crust over water."
The mathematical analysis in this paper of the effect of the tidal forces
and the effect of thermal convection in the solid crust lead the authors
to two possibilities for the present situation on Europa: "Provided
that the orbital eccentricity has been near its present value for most
of Europa's history, an equilibrium configuration could exist in which
the heat generated by tidal dissipation in a thin ice crust (<10 km)
is balanced by thermal conduction to the surface. The tidal dissipation
would greatly exceed the heat generated by radioactive elements. A deep
(~ 90 km) ocean would exist beneath the ice crust." or "Another
equilibrium configuration exists in which the entire H2O
mantle is frozen, and in which tidal dissipation augments (but probably
does not exceed) radioactive heating." "
The authors also consider the possible fracturing of the icy crust.
They write: "In the situation in which tidal dissipation is able to maintain a thin, stable crust, one might ask whether or not such a crust would remain intact." Based on a mathematical analysis comparing the tensile
strength of the icy crust with the stress which it is subject to by the tidal forces, they conclude that: " Tidal stresses may be great enough to fracture a thin ice crust, thereby permitting water to evaporate and precipitate
elsewhere on the satellite." As they explain, such fractures would
expose the underlying water to the near vacuum conditions on the surface and would result in vigorous boiling of the water. However, they point out that if a liquid water mantle does become frozen, the crust would be forced to expand, and this would also result in fractures on the surface.
The scientists who studied the images obtained by
Voyager 2 published a summary of their findings and interpretations in the November, 1979 issue of Science
(in The Galilean Satellites of Jupiter, Voyager 2 Imaging Science,
authored by B.A. Smith and 21 other members of the imaging team).
Two pages are devoted to Europa. After discussing the hypothetical
theoretical models of Europa' s interior which predict a layer of liquid
water, the authors suggest that the darker regions on the surface
of Europa*may be areas where the rocky
core comes rather close to the surface of the ice (within ~ 10 km).
If this is so, they argue that the total depth of a liquid water layer
should be less than that predicted by the theoretical models ( ~50 km)
because otherwise the topography of the core would be unusually great for
a body of Europa's dimensions. This leads them to conclude that the
density of the core would be low and therefore that it might contain significant
amounts of water. The authors also discuss the dark linear markings in
the brighter regions of Europa's
surface*, suggesting that these might have been caused by expansion
of the icy crust due to freezing of an early ocean, creating fractures
in the ice which could be filled by fluids from below and which are now
visible as dark markings. The widths of these marking as estimated
from the Voyager 2 images indicate that the amount of expansion must be
about 5 to 15 percent of the surface area. In order to explain this large
amount of expansion, they propose that a thin (~50 km) ocean might have
been produced over a period of time from water outgassing from the core
and that the frozen crust was forced to adjust to the increasing volume
of that ocean.
In the January, 1980 issue of National Geographic, there is a marvelous
article by Rick Gore entitled What Voyager Saw: Jupiter's Dazzling Realm.
It is a long article, filled with beautiful Voyager photos and many quotes
from various scientists involved in the Voyager mission about what they
expected to see and what they learned from the mission, and it manages
to convey
the excitement surrounding the new discoveries. Concerning
Europa: "Europa, however, was Voyager 2's star. The
scientists were predicting that water-rich Europa could be heated by the
same kind of tugging as Io - albeit much less so. "We were hoping to see
Old Faithful going off," said geologist Hal Masursky. Voyager 2 saw
no geysers - but its resolution was only good enough to detect mammoth
ones." Commenting about Europa's remarkable flatness
and lack of craters , which led scientists to conclude that Europa has
a relatively young surface: "An Io-like tidal heating may
indeed be keeping the crust of Europa plastic and the ocean beneath either
liquid or soft ice. But no one can do more than guess at what mechanisms
Europa uses to erase its craters."

In recent years there has been considerable speculation about the possibility
of life on, or within, Europa. This has been spurred by the increasing
evidence that an ocean might really exist under Europa's icy crust. One of
the ideas that has frequently been suggested is that geothermal energy,
which is the primary source of energy supporting life in certain deep sea
regions here on Earth, might also provide the needed energy source for life
at the bottom of an Europan ocean. The first discovery of those deep-sea
communities of life on Earth was in 1977 - Robert Ballard's expedition in
the Alvin to the rift zone deep under the Pacific near the Galapagos
Islands. That discovery was the subject of an article in the October, 1977
issue of National Geographic (Oases of Life in the Cold Abyss, by J. Corliss and Ballard), and a later expedition led to another article in the November, 1979 issue (Return to Oases of the Deep, by Ballard and J. Grassle). Both articles are filled with intriguing photographs
showing certain kinds of giant worms and clams, and other exotic creatures that thrive in
those regions, seemingly without any dependence on sunlight. (A, B, C).
Those
discoveries on Earth, together with the theories of possible oceans on
Europa, Ganymede, and Callisto, inspired some individuals to already make the link in
the late 1970s. One notable example is the physicist Gerald Feinberg, who
came to this idea early in 1979, and realized that a theory that he had
developed with the biochemist Robert Shapiro (presented in their book
Life Beyond Earth,
published in 1980) might explain how life could develop deep in those Galilean oceans. The essential requirement for their theory would be that
the internal heat from the rocky core of those bodies
reach the ocean in a concentrated form, such as in a volcanic eruption
or an upwelling of hot gas, which would create the necessary "deviation from
equilibrium."
On June 19th and 20th, 1979, the conference "Life in the Universe"
took place at NASA's Ames Research Center. Benton Clark gave a lecture Sulfur: Fountainhead of Life in the Universe at that conference in which he discussed the biochemistry of those deep-sea vent communities discovered on Earth, pointing out that they do depend indirectly on sunlight: Photosynthesis near
the surface of the oceans produces the oxygen that those communities require.
Clark then explained how sulfur could play the role of oxygen, and that deep-sea volcanic emissions could potentially provide all the necessary
ingredients for a self-sustained ecosystem. In the final
part of his lecture, Clark raised the possibility that life might exist in
undersurface oceans on the icy satellites in our Solar System, including Europa, Ganymede, and Callisto in particular.
In January, 1980, Richard Hoagland published a long article entitled The Europa Enigma in the magazine Star & Sky. It concentrates
specifically on Europa, and was inspired by the images of Europa provided by
the Voyager mission in July, 1979, and by the theory that tidal heating might maintain an ocean on that body under its crust of ice. At the end of the article, Hoagland also makes the
link with the discoveries of ecosystems of ocean bottom
life near deep sea vents, and suggests that Europa "has all the ingredients to permit the existence of similar internally nurtured oases of life."
We will discuss the ideas of these individuals in more detail below, and also some even earlier speculations about life in Galilean oceans going back
to 1975, before the discoveries made by Ballard's expedition in 1977. The
possibility that some of the satellites of Jupiter might have an ocean
became somewhat widely known in the 1970s. Isaac Asimov mentions that
Ganymede and Callisto might have oceans under a thick crust
of ice in his book Extraterrestrial Civilizations, published
in 1979. In New Worlds for Old (also published in 1979),
Duncan Lunan devotes a good part of his chapter about Jupiter to the Galilean
moons. He writes that "Io's surface might have extended salt
beds, perhaps deposited by evaporation of water from below ground in the
past. If so, Europa and Ganymede might still have sub-surface "oceans."
(Both Ganymede and Europa seem to have surface water ice.) "
A little later he writes that "On 4 May 1976 Ames Research
Centre sent us the most staggering release to date. Ganymede, it
seems, may be almost all
water - a single droplet larger than Mercury,
encased in rock and ice." . In his book cited above, Asimov raised this natural question: can life develop in a "region of eternal darkness, sealed away
from the rest of the Universe by an unbroken miles-thick layer of ice?
" Guy Consolmagno, who worked on the theoretical models of oceans on Europa, Ganymede, and Callisto with John Lewis at MIT, included an appendix in his
1975 Master's thesis Thermal history models of icy satellites where he suggested that Europa could have the
beginnings of organic chemistry if the rocky core is as rich in carbon as some of the primitive meteorites. He noted that the core would be in intimate contact with the large mantle of water and that geological evolution, like lava flows, might occur, producing a geochemist's delight of possible reactions, easily comparable to the complexity of Earth's salty oceans. He concluded his thesis by writing "... we stop short of postulating life forms in these mantles; we leave such to others more experienced than ourselves in such speculations."
In his book Brother Astronomer - Adventures of a Vatican Scientist, published in 2000, Consolmagno gives an account of a conversation that he had with Carl Sagan just before he was to present his work on the models for oceans on the Galilean satellites at a conference about Jupiter in 1975. Consolmagno suggested to Sagan that such oceans might be places to look for life. Sagan responded quite skeptically, saying that "Life needs energy, sunlight. How are you going to get sunlight through a thick crust of ice." In the question & answer session after his presentation, Consolmagno mentioned his idea that there was a possibility of life in the Galilean oceans, immediately adding: "But Dr. Sagan pointed out, there's no energy source for them - no sunlight down there." Interestingly, in his long article
The Solar
System Beyond Mars: An Exobiological Survey, which appeared in
Space
Science Reviews, vol. 111, in 1971, Sagan himself included Europa, Ganymede, and Callisto
in a list of bodies in the outer solar system which he believed offered
"interesting
exobiological
opportunities.". Just the presence of water in the form of ice or snow
on the surfaces of those bodies led Sagan to make that remark. Arthur C. Clarke made a similar remark about Europa and Ganymede in 1974 in his essay Closing
in on Life in Space, writing that they possess "at least
one of the preconditions for life: the presence of water
"
[in the form of frost or ice].
Duncan Lunan's book New Worlds for Old is based on discussions and
public lectures sponsored in the mid-1970s by ASTRA-the
Association in Scotland to Research into Astronautics. This group
was founded in 1953 and is still very active. Lunan's book is densely-packed
with perceptive and well-informed speculations about our Solar System.
Chapter 9, which is devoted to Jupiter, has a lot to say about the
Galilean moons. Speculation about how life might begin to develop on the
Galileans is left to the last two pages of that chapter - just a few provocative
paragraphs. Here are some quotes: "At ASTRA, Robert
Shaw suggested that life might be evolved following the passage of a comet
through the Jovian system: now that we know that comets have immense hydrogen
haloes, we might expect that when one interacts with the thin atmospheres
of the Galileans it generates powerful electric storms, perhaps synthesizing
complex molecules as lightning may have done on the primeval earth.
"
"With the storms, and the successive freezing and thawing of the atmosphere
during eclipses, such compounds might find their way into liquid reservoirs
below ground - or interact more slowly in the snow. Any life which comes
into being has to survive the radiation belts, however ; if
sheltered by a crevice, it must have some energy source such as volcanism
to take the place of sunlight."
Lunan offers his own speculations based on a hypothesis of A.T. Lawton
presented at ASTRA : " . . . in discussion he [Lawton]
suggested that there might be a ring of dust surrounding the Solar System,
steadily draining into it. In that way the Galilean moons could have
been subjected, over a long period, to an infall of interstellar
dust [1,
2,
3]
rich in heavier elements from supernova explosions. Such surface dust might
well give the Galileans complex chemistries. Everything then depends, again,
on whether storms transfer such compounds into crevices - to shelter from
the radiation, perhaps to trickle down into warmer regions." "Underground
water would greatly increase the chances that life might evolve.
In a liquid medium, the required chemical interactions are far more likely
to occur and primitive organisms have better chances to spread, survive
and evolve. There is water ice on Ganymede and Callisto, at least ;
their densities are low; is it too speculative to imagine internal
heat and subterranean lakes, even seas ? "
After mentioning that Ganymede might be almost entirely water, Lunan
writes: "Imagine looking out of a window at that, orbiting
it, landing on it . . . . Imagine penetrating the crust,
sending the first bathyscaph down . . . . Will the water be clear
or cloudy? How far will the lights carry? Could there be life
in that unimaginable blackness, clinging to pockets of radioactive
heat on the underside of that extraordinary shell ?" Duncan
Lunan ends the chapter by writing that "once terrestrial
industry is established in Earth orbit,there are . . . probably
not more than a hundred years to pass before there are marine biologists
on Ganymede."The
book Life Beyond Earth: The Intelligent Earthling's Guide to Life in
the Universe is quite different. The
authors Gerald Feinberg and Robert Shapiro take a systematic approach,
attempting to develop a very broad perspective on how and where life might
originate in the Universe. One long chapter is devoted to the general
conditions which are essential for life; another chapter looks at
a variety of possible chemistries which could serve as a basis for life.
Their ideas were presented in a lecture entitled Possible Forms of Life
in Environments Very Different from the Earth given by Feinberg at
the conference Extraterrestrials: Where Are They?
which took place at the University of Maryland in November, 1979.
The authors offer the following definition of Life on which their entire
discussion is based:
"Life is fundamentally the activity of a biosphere.
A biosphere is a highly ordered system of matter and energy characterized
by complex cycles that maintain or gradually increase the order of the
system through an exchange of energy with its environment." For
example, they consider the biosphere of Earth to include the totality of
all living things together with all nonliving things which enter into their
metabolic activity. The authors propose the following three
conditions for life to originate and develop: "A flow
of free energy. " "A system of matter capable of interacting
with the energy and using it to become ordered." "Enough time
to build up the complexity that we associate with life." On
Earth, these conditions are provided by light and chemical energy,
nucleic acids and proteins, and the eons of time of a relatively stable
environment.
The last third of the book is devoted to taking the
reader on a tour of the Solar System and beyond from the point of view
of the general principles that the authors have already fully described.
Their discussion of the Galilean satellites is based on the theoretical
models proposed by Consolmagno and Lewis in 1976.
They state that Europa, Ganymede, and Callisto are fairly
similar to each other, and so they just focus the discussion on Ganymede
because it is the largest. Here are some quotes from that discussion.
"Beneath the ice crust, which is fifty to
one hundred kilometers thick, lies an enormous ocean, five hundred kilometers
deep. The situation resembles that of our Arctic ocean, but the Ganymede
ocean is vaster. There is twenty-five times as much liquid water under the ice of Ganymede
as on all of Earth. Below this ocean is the rocky core, at a temperature
that varies from 25o C at the bottom of the ocean to several
thousand degrees at the center of Ganymede""Neither Ganymede's ice surface nor its
ocean is pure water. The water contains dissolved impurities of many kinds,
just as Earth's oceans do. The precise chemical form of these impurities
is unknown, but they may well contain the same elements and simple compounds
present in the primitive oceans of our planet. Furthermore, Ganymede's
ocean has probably existed in its present form for several billion years.
Therefore, this ocean satisfies two of the conditions necessary for life
- a suitable material base and enough time for prebiotic and Darwinian
evolution to take place.""The crucial factor which may determine
whether life exists in the ocean of Ganymede is whether a suitable energy
source has existed to drive the matter away from equilibrium. The water is shielded from the feeble
sunlight of Ganymede by the ice crust. It is hard to imagine any useful energy getting through to the ocean from above. However, there is another direction from which energy
can reach the ocean - underneath from the hot rocky core. The same radioactive decays that originally melted Ganymede
are still producing heat in the core, and this heat works its way out to
the ocean in various forms. In
order to be of use as an energy source for life, the internal heat must
reach the ocean in a concentrated form, such as in a volcanic eruption
or an upwelling of hot gas. Otherwise, the heat will just raise the
overall temperature at the bottom of the ocean slightly, and will not be
available as free energy for life. In our
present state of knowledge of the internal workings of Ganymede, we
cannot be sure whether rich concentrated energy sources will exist under
its ocean. Analogies with Earth
would suggest that a significant fraction of the energy would emerge in
concentrated form at local hot spots, and at those spots, the deviations
from equilibrium that are the beginning of life may occur. (The places
on the ocean bottom on Earth where hot springs emerge are rich sites for
living creatures. These areas derive their primary energy source from minerals
in the hot springs, rather than from the Sun.)" Of course, as the
authors indicated , these speculations apply equally well to Europa and
Callisto, assuming the validity of the Consolmagno-Lewis models for these
bodies. The last parenthetical quote is referring to the discoveries of the Ballard expedition in 1977 of the thriving communities of life at deep ocean vents in the Galapagos Rift zone.
The conference "Life in the Universe"
took place at NASA's Ames Research Center in June, 1979. It was a relatively large conference,
attended by about 150 people, including scientists from NASA and from the
academic world. The lectures covered a wide range of topics,
from fundamental exobiological questions to SETI, and were published
in 1981 in a volume edited by J.Billingham, also named Life in the Universe.
Two of the lectures delivered at that conference mentioned Europa, Ganymede,
and Callisto specifically. We have already mentioned Benton C. Clark's
lecture entitled Sulfur: Fountainhead of Life in the Universe.
In the beginning summary of the written version of his lecture, Clark
writes: "Sulfur is ubiquitous in the Universe and essential
to all life forms that we know. It supports the chemoautotrophic
way of life and the photosynthetic. It may inhabit niches we cannot
imagine, and the life zone about a star may therefore be wider than now
estimated." "Although it seems most likely that liquid
water and organic compounds are essential ingredients for the vast majority
of (if not all) biotic systems in the Universe, it will be my theme that
sulfur compounds may be of equivalent rank and may well permit the proliferation
of life in certain environments not otherwise considered hospitable."
Early in the paper, the author discusses the discoveries
made in 1977 with the manned submersible Alvin: "The mission
was the geological exploration of ocean-bottom thermal springs on the 2.5-km-deep
center of the Galapagos rift zone. Hydrothermal vents were indeed
found, and although these are of considerable geochemical and geophysical
interest, the most important discovery was the existence of previously
unknown species of animals whose communal life is dependent on the primary
productivity of sulfur-oxidizing bacteria."
Much of the paper describes
in some detail the role of sulfur in the chemistry of the Universe and
our Solar System, its role in planetary evolution and especially
in the chemistry of life. Later , in a section entitled "Galapagos
Discovery", he writes: "The finding of small, isolated,
and more or less complete ecosystems at the mouth of active hydrothermal
submarines vents is important since such systems do not depend on photosynthesis
for their primary productivity." He points out that those
deep sea vent communities do require dissolved oxygen in the seawater,
which is generated mainly by photosynthesis near the surface. Then
he writes: "It is interesting to speculate, however,
that submarine volcanic emissions could provide all the necessary
ingredients for a self-sustained ecosystem." He suggests some
alternative chemistries which such volcanic emissions could support,
including one specific set of linked, sulfur-based, energy-yielding chemical
reactions which could be a basis for a biological system. The
last section is entitled "Other Worlds." Here are some quotes:
"The assumption that only Earth-like environments qualify as CHZs
[continuously habitable zones] is not a secure one. We have been biased
by the idea that photosynthesis is of such fundamental significance that
advanced biotic systems can persist only in environments coupled to illumination.
However, the existence of niches only very indirectly coupled with the
solar photon flux, such as the Galapagos vent communities, other benthic
and marine mud ecosystems, and salt-marsh environments, emphasizes that
the most fundamental requirement is energy flow to provide recycling of,
or a fresh supply of, chemical potential energy."
"The basic requirements of life may simply be (1) a flux of energy,
(2) a stable temperature regime compatible with the biochemistry of the
organisms, (3) a liquid milieu, and, of course, (4) an initial
supply of building-block elements, such as C, H, N, O, P, S, and
transition metals. These elements need not, in principle, be replenished
since they can be recycled. Under these conditions, certain
non-Earth-like environments may not only be conducive to life, but may
be available in far greater abundance in the Universe." "Consider H2O-rich
bodies. In our Solar System, this includes not only Earth, but quite possibly
Mars and Triton, and certainly Ganymede, Callisto, and Europa. Liquid
water does not exist at the surface of any of these bodies, except Earth,
but we should not discount the existence of "buried" liquid water reservoirs.
" Clark briefly summarizes the possible sources of such undersurface
liquid water reservoirs, including tidal effects such as those responsible
for volcanic action on Io. "Regardless of the manner in which
they are formed, there is good reason to consider buried liquid water reservoirs
as possible life-supporting environments. The probable availability of
dissolved salts, including sulfur compounds, and the existence
of energy flow, in the form of planetary heat flux, satisfy the life-supporting
requirements listed above. Coupling to stellar or planetary luminosity
may be completely unnecessary." "Sulfur is
ubiquitous and probably plays several important roles in any exobiological
organization. Although it can participate in photosynthesis, it also
permits the chemoautotrophic way of life. Habitable zones include not only
the surface ocean environment, but also the much more probable subsurface
oceanic regions. Earth-like environments as abodes for life may be the
exception rather than the rule. Occupation of the much more abundant buried
zones is possible, and these should ultimately become an object of exploration.
Whether such environments can support life long enough and at a sufficient
level of activity to permit the evolution of highly encephalized forms
(intelligent life) is conjectural.
"
Clark concludes his paper by
mentioning the research fields that would have a bearing on the speculations
he has presented, including especially the study of large planetary satellites
such as the Galilean satellites of Jupiter, their thermal history and the
question of how long subsurface liquid water might have existed on such
satellites. Another question that he emphasizes is whether totally
chemoautotrophic ecosystem are likely to survive on a large enough scale
and for long enough time periods to permit higher evolution.
Richard
C. Hoagland's article The Europa Enigma can be found in its entirety on the Enterprise
Mission website. It was widely publicized at the time by Terence Dickinson, the editor of the magazine Star & Sky in which the article appeared. He issued a news release which prompted reports about Hoagland's ideas in numerous newspapers. This article inspired
Arthur C. Clarke to make Europa and the possibility that life might exist there one of the themes for his novel 2010:
Odyssey 2. Hoagland presents his own theory of how complex organic chemistry, which could be the precursor of life,
might have come to exist in a possible ocean on Europa. His starting point is the often-stated analogy
between Jupiter and its system of orbiting moons and a sun with its system
of orbiting planets. Recounting his thoughts as he watched the images
from Voyager on the TV screens at JPL, he writes: "But that
night, as we swept through the Jovian system and Voyager returned image
after image of vastly different worlds - each Jovian satellite more stunning,
each more intriguing than the last, each a place which would be a major
planet
if it orbited the sun - it was then that the dry, academic rhetoric about
"miniature planetary systems" suddenly leaped off the screens and became
a set of mind-expanding possibilities." He then continues:
"At one time, so the theorists tell us, that image was far more accurate.
The newly-forming Jupiter, accreting out of the primeval solar nebula of
swirling dust, hydrogen, helium, and traces of other elements, resembled
in every essential respect a newly-forming star. It glowed - with
a fierce ruby light - radiating as much energy as a traditional main sequence
red dwarf star, about one ten-thousandth of the current sun. ""
The critical
difference was, of course, that the sun shines by nuclear energy resources
and Jupiter was drawing on far more limited reserves, the transformation
of the gravitational energy of its collapse into waste heat." "But,
in between Jupiter's moment of "ignition" and its "fading" there must have
been a window, one brief slice of time when Europa basked in energy as
rich as any streaming out across the orbit of Earth . . . or Mars."
"It was then that Europa must have had real oceans and cloud-puffed
skies with gentle rains or slashing hurricanes to turn those sparkling
seas to froth before a storm." "And yet Europa's fate was
sealed. It died while Earth itself was still cooling toward the moment
when its first oceans could be born. Jupiter continued to evolve, growing
small and dim." "In a geological instant the waters that splashed
across this oh-so-young Europa froze and a vast satellite-wide ocean was
suddenly transformed into a shimmering expanse of ice, reflecting for eternity
the faded "sun" of its brief fling at life. Jupiter is now held motion-less
above one shining hemisphere by Europa's now tide-locked synchronized rotation."
Hoagland then discusses at length one way in which organic molecules
might come to be synthesized in the early atmosphere of Europa, which he
suggests might have been similar to that of Earth in its early history.
He argues that this early atmosphere would have been very heavily ionized
and that it would be lost into space over a period of millions of years,
creating a ring along its orbit of heavy ions. He then writes that,
as a result, interaction with the primeval magnetic field of Jupiter
would create intense electric currents between the poles of Europa and
the Jovian photosphere. This would have led to "heating of the atmosphere
above the poles", "massive superbolts of lightning,
even in clear air." "And one more thing: organic
synthesis reactions between the major and minor constituents within
this atmosphere!" He also mentions several other sources of
organic synthesis. "The result should have been a veritable
rain
of molecules falling from the skies above this youthful planet, everything
from alcohols to penultimate
amino
acids." In this way, before a frozen crust covers
the surface of Europa, an ocean rich in organic compounds could be produced.
After summarizing the ideas of Cassen, Peale, and Reynolds concerning the
possibility that tidal forces exerted on Europa might maintain a sea of
liquid water under its icy crust, Hoagland continues: "If true,
the continued existence of the solar system's deepest planetary ocean -
across 4.5 billion years - presents us with a staggering set of possibilities,
including the independent evolution
beyond those pre-organic chemicals
and acids into the object of our centuries-long quest: the solar system's
second world of life." He then discusses the extensive, dark cracks
covering Europa's surface, suggesting that the presence of these markings
indicates that the ice crust is thin and that their darkness might be due
to "radiation polymerized organic molecules brought up from far
beneath, staining the surface crust for miles beyond the actual fracture
of the ice." "Even if only relatively simple molecules, the sudden
exposure to raw solar ultraviolet and the high energy radiation background
of the surface would inevitably cross-polymerize these compounds into different
combinations, quite likely producing brownish stains along the fissure
identical in chemistry to those produced in all those laboratory flasks!"
Proposing a description of Europa as a "pressure-cooker planet,"
Hoagland explains: "Europa's immense good fortune was that Jupiter
did
die leaving it with a perfect atmospheric seal against the loss of all
its volatiles. With the establishment of the miles-thick icebound crust,
volcanic activity from the ocean floor would have continued for several
million, if not billions of years, totally uncaring that the products -
water vapor, carbon dioxide, ammonia, nitrogen, surphur, etc. - were now
trapped beneath a lid, represented by the crust fifty miles above."
"The chemical and organic evolution of those organic molecules produced
during Europa's first few million years could have continued under that
canopy of ice, augmented by a variety of energy resources, and aided by
that one commodity every exobiologist agrees is quite essential: Time -
about 4.5 billion years of it." "And the most intriguing clue
that this process at this moment is occurring, is those peculiar
surface markings covering Europa like no other planet in the solar system." "Finally,
what if evolution on Europa did continue, past microbes living in
their anaerobic ocean, past organisms using merely energy of fermentation?
Suppose the combination of several billion years and the unique environment
fostered - forced - the evolution of much more complicated organisms?
Could there, in fact, be the equivalent of plesiosaurs swimming in the
forever dark beneath Europa's blinding landscape: the evolutionary cousins
of the great blue whale; the intellectual equivalent of dolphins, or ourselves,
locked in that icy prison, forever trapped in orbit around their almost
star, who have never seen the real stars and have no way of knowing anything
beyond their deep, dark, liquid water?" Near the end of his
article, Hoagland discusses the discoveries of ecosystems of ocean bottom
life near deep sea vents, which are based on chemical synthesis rather
than sunlight, and the relevance of these discoveries for Europa: "Europa's
ocean, according to the line of reasoning in this article, potentially
has all the ingredients to permit the existence of similar internally nurtured
oases of life."

With
the wealth of discoveries and data obtained by the two Voyager missions
of 1979, a conference called "The Satellites of Jupiter" was soon
organized and took place in May, 1980, sponsored by the Institute
of Astronomy of the University of Hawaii and supported by NASA, the International
Astronomical Union, and several other scientific organizations. There
is an excellent article in the proceedings of this conference by Cassen,
Peale, and Reynolds, entitled Structure and Thermal Evolution of the
Galilean Satellites, which discusses in detail the then current ideas
about the internal structure of the Galilean moons, and gives a rather
thorough account of the history of those ideas. Concerning the possibility
of the existence of oceans on Ganymede and Callisto, the authors write:
"The
water mantles of both satellites are probably completely solid, unless
they are significantly contaminated by dissolved salts or ammonia. The
major uncertainties in thermal models of these bodies are due to uncertainties
in the creep properties of ice and the degree of contamination of the water
mantles. " Concerning Europa: "Tidal heating undoubtedly
has contributed significantly to Europa's thermal history, but whether
it is enough to maintain liquid water depends on the viscosity of the ice,
the history of the orbital resonances, and the quantities of impurities
in the ice." But the authors are clearly somewhat less optimistic
about the possibility that Europa might still have a liquid water mantle
than in their earlier paper of 1979 (Is there liquid water on Europa?).
A small but crucial mathematical error in that paper weakens considerably
the argument presented there. The authors discuss this error and its implications
in another paper in the November, 1980 issue of Geophysical Research
Letters, entitled Tidal dissipation in Europa: A correction.However,
in a brief paper entitled Liquid water and active resurfacing on Europa
,
which appeared in the January, 1983 issue of
Nature, Cassen,
Peale, Reynolds, together with S.W. Squyres present some additional,
new arguments which then strengthened the case for the existence of a liquid
water ocean on Europa. They first reexamine the principal heat sources
that could effect the surface temperature, namely the heat produced by
radioactive decay in the core, by tidal forces acting on the icy crust,
and by tidal forces acting on the core itself. This last source was
not considered in the earlier paper of 1979 by the first three authors.
Under certain hypotheses, the authors argue that their calculations are
at least consistent with an H2O-layer (liquid
water and ice) many tens of kilometers thick having an icy crust about
16 kms thick. But they point out that the calculations are very sensitive
to even slight changes in the hypotheses made, and then discuss some observational
evidence. Based on the paucity of craters on Europa, the authors
arrive at an estimate of the viscosity of the surface ice. They then
argue that this viscosity requires some kind of insulating blanket, such
as an extensive layer of frost on the surface. The authors then write:
"Fracturing
of a thin ice crust over liquid water could provide such a blanket. Water
exposed by fracturing would not flood the surface , due to the buoyancy
of the crust, but would boil, producing vapour that would condense as frost
over a large area. Frosts typically have very low densities and thermal
conductivities, and could provide the insulation required. An insulating
layer could also result in an average crustal thickness much less than
the mean value of ~16 km calculated for conduction in solid ice alone."
The
authors conclude by presenting observational evidence in favor of the presence
of an extensive frost layer.
One of the crucial requirements for the existence of life in an undersurface
ocean on Europa would be adequate sources of energy. Precisely this
question is studied in the paper On the Habitability of Europa by
R.T. Reynolds, S.W. Squyres, D.S. Colburn, and C.P. McKay, which
appeared in Icarus , vol. 56, in 1983. In their introduction,
the authors write: "Given the indications of a substantial liquid
water ocean on Europa, and the affinity of life forms on Earth for an aqueous
environment, it is perhaps appropiate to investigate the possibility of
Europa's habitability. In this paper we consider the type of environment
that might exist in an ocean on Europa and the suitability of that environment
as an abode for life. The description of a hypothetical ecosystem on Europa
would entail a knowledge of the environment far beyond anything currently
available. It is possible, however, to address the much more limited question
of the availability of basic requirements for living systems. These include
(1) a proper physical environment (appropiate temperature, pressure, etc.),
(2) long-term stability of that environment, (3) necessary biogenic
elements, and (4) adequate energy sources." The authors then
point out that the presence of a liquid water ocean would imply that conditions
(1) and (2) are satisfied, and argue that quite reasonable assumptions
about the formation of Europa imply that (3) is satisfied too. For
these reasons, the paper concentrates specifically on the question of whether
biologically useful energy sources should exist on Europa, examining in
turn thermal energy, solar energy, and electrical energy.
Thermal Energy. An ocean on Europa would be heated as a result of
radioactive decay in the core, tidal forces on both the core and the icy
crust, and the release of stored energy from an earlier period of higher
heating. The authors argue that this energy would probably not be
useful for supporting life. Although there would be temperature differentials
in such an ocean (e.g. an increase in temperature with depth), life
forms which depend on such temperature gradients as an energy source would
probably have to be kilometers in length. The authors then discuss
the possibility of concentrated sources of heat existing on Europa, analogous
to the deep sea vents and hot springs that were discoverered in the late
1970s here on Earth. The calculation of the amount of heat energy
produced in Europa's silicate core by radioactive decay and tidal forces
does not by itself imply that ocean-bottom volcanic activity is very probable.
Nevertheless, combined with other factors, the authors write that such
activity is at least a possibility and that current data and modeling techniques
are just not good enough to assess the chance that such concentrated energy
sources should exist.
Solar Energy. "We assume that solar energy will only reach
the liquid water where the ice crust has been recently fractured. An important
quantity is therefore the annual amount of Europa's surface area over which
liquid water is exposed to sunlight by crustal fracturing."
Such fracturing may produce a layer of frost on the surface. Also,
the exposed liquid water will begin to freeze rather quickly. By
making some reasonable estimates of the thickness and density of the frost
layer, the authors arrive at an estimate of 5 square kilometers for the
total area of Europa's surface where the underlying liquid water would
be exposed to sunlight during one year. Then they can estimate the amount
of solar energy this exposure can provide to an undersurface ocean on Europa,
arriving at a figure of 2x1022 ergs per
year.
Electrical Energy. "Another possible source of biologically
useful energy could result from the motion of Europa through Jupiter's
magnetosphere." The authors argue that the induced electrical
current will tend to flow through the liquid water ocean (if it exists)
from one pole of Europa to the other and the magnitude of this current
depends on the conductivity of the ice crust at the poles. They conclude
that this source of energy would only be significant when the underlying
liquid water ocean is actually exposed at the poles. The authors
raise the possibility that such an electrical current could create enough
heat to permanently maintain areas of exposed water at the poles, which
would also allow significantly more solar energy to reach the liquid water.
There is a considerable literature still to come after 1983. One
excellent guide to more recent articles and links about Europa, the possibilities
that this moon of Jupiter offers, and all the related issues is the essay
by Charles Tritt:
Possibility
of Life on Europa. For some references and links concerning
the situation as it seems at the present time, the reader should
look at the Commentary .